Studies are in progress on the membrane properties and contractile responses of intrafusal muscle fibers in isolated cat muscle spindles. Intracellular micropipettes are used for recording and current passage. Contractile responses are recorded through an inverted microscope with differential interference contrast, using high speed cinematography. Spontaneous miniature endplate potentials are also being recorded from intrafusal muscle fibers. These studies aim towards an understanding of how nuclear bag and nuclear chain fibers respond to their motor innervation and modify the response on sensory endings to muscle stretch.